A conventional optical oximeter is known as an apparatus for measuring oxygen saturation in arterial blood based on light transmitted through the finger, the ear or the like of a person to be examined, when light is applied thereto.
U.S. Pat. No. 2,706,927 discloses an apparatus for evaluating oxygen saturation based on measured values of absorbance of each of two different wavelengths in two states, i.e., a state in which the ear is pressed and congested and a state in which the pressure on the ear is relieved. The measured value in the congested state is baased on only absorbant components other than the blood and the measured value in the non-pressed state is based on both of the blood and the other absorbant elements. Therefore, the absorbance of only the blood should be indicated by camparing values read or measured in the two states. However, the precision of the measured value would be lowered because all the blood cannot be removed by pressing the ear and because optical connections between the ear and the optical apparatus vary. In addition, the influence of the absorbant components due to differences in color of the skin and the skin thickness, for example, can differ considerably dependent on the respective persons to be examined and, accordingly, it is necessary to effect a calibration for each person or each measured value.
U.S. Pat. No. 3,638,610, discloses how to avoid described defect by utilization of measured values of absorbance based on a plurality of wavelengths of light. Similarly to all conventional apparatuses, the good result obtained by the apparatus of U.S. Pat. No. 3,638,610 depends on an increase of perfusion in the living body examined. For that reason, the perfusion in the living body is made to be as close as possible to the arterial blood as possible. The perfusion can be increased artificially until an accurate result can be obtained. However, such method is often unfavorable or very difficult dependent on the conditions of the person examined.
Japanese Patent Laying-Open Gazette No. 88778/1978 discloses an oximeter having the below described features. Light of one wavelength and light of another wavelength are applied successively to the fingers, the earlobes or other parts of a living body. The known oximeter comprises photodetector means which generates a first electric signal proportional to part of the light of a wavelength absorbed in such part of the body and generates a second electric signal proportional to part of light of another wavelength in that body part. When the heart sends a larger quantity of blood to the artery tissue than during a heart pause, a larger quantity of blood exists in that part of the body and accordingly the lights of the two wavelengths are more attenuated than during the heart pause. Consequently, the first and second electric signals have peaks of the maximum and minimum values in one pulse period of the heart. The difference of the maximum and minimum peak values entirely depends a pulsating current of blood, while the pulse period is not at all influenced by the absorbant component which attenuates light by a given quantity.
However, a measurement is not permitted in a body part where an artery blood current is not obtained or in a body part where a cuvette necessary for detection of transmitted light cannot be attached.
Japanese Patent Laying-Open No. 51785/1977 discloses a reflection type oximeter which can be attached to a part of a living body without a cuvette as is required in the above described examples. However, the oximeter of Japanese Patent Publication 51785/1977 is used in principle for detecting of a pulsation component and accordingly it is impossible to make measurements if the pulsation component is not obtained.
Japanese Patent Laying-Open No. 160445/1984 discloses an oximeter wherein a pulsation component of the artery blood current is detected as a change of a transmitted light component of the light applied to the tissue, whereby an oxygen saturation in the arterial blood is measured. Consequently, the following disadvantages are involved.
Such an oximeter is incapable of making measurements in a part or a state where a pulsation component does not exist. The measured results are only an oxygen saturation degree and a quantity of hemoglobin and the apparatus is incapable of measuring a tissue oxygen saturation including and providing information on venous blood serving as an index representing metabolism of the tissue. Since the oximeter of Japanese Patent Publication 160445/1984 utilizes transmitting and absorbing functions of the mechanism, it can be attached only to a part used as an optical cell. In addition, since a transmission path of light is not clearly known, it is not clear to which part (volume) the detected information pertains. Further, noise occurs due to sway or vibration of the sensor.